A great option I've found (besides Dollar Tree) is Forever 21! They have super cute glasses for under $6. LOVE IT!
These are only $1.80! I love the color - so summer - and the aviator style looks good on just about everyone!
$3.50 - Basic, black sunglasses. The frames are stylish and make the look a little less boring.
$5.80 and super cute! So catty. :) Fierce!
$5.80 - These have a cool shape. They're black/brown - go with any outfit.
These are just a few options! They have many, many more on their website and in stores. :)